The present invention relates generally to the field of telecommunications, and more specifically, to the field of intermodulation spurious response attenuation in cellular telephones.
Intermodulation (IM) signals are problematic in radio communication systems, such as cellular communication systems, since they can interfere with communication signal processing under certain circumstances. IM signals are extraneous signals produced when two or more interfering signals combine and are, for example, mixed in a non-linear medium such as the non-linear operating region of a semi-conductor device such as an amplifier. One situation where this problem occurs is where a mobile station operating in a digital mode is far from a digitally transmitting base station but very close to an interfering analog transmitting base station. In such a situation, the mobile station receives very strong interfering analog signals relative to the desired received digital signals. These very strong interfering analog signals often result in the production of intermodulation signals that fall into the frequency range of the desired digital signal frequency spectrum, the effects of which are to increase the communication errors at the mobile station, potentially causing calls to be dropped.
While the problems of IM distortion are not isolated to one particular type of radio communication system the wideband spread spectrum cellular communication system is one type of radio communication system which is sensitive to the problems of IM distortion. IM distortion is currently addressed in section 9.4.3 of the standard promulgated by the Telecommunications Industry Association (TIA) and the Electronics Industry Association (EIA), TIA/EIA/IS-98, entitled "Recommended Minimum Performance Standards for Dual-Mode Wideband Spread Spectrum Cellular Mobile Stations". Currently, the standard calls for a mobile station receiver performance with a 1%, or less, frame error rate (FER) when receiving an input signal at -102 dBm (very low) on the assigned frequency channel in the presence of two continuous wave (CW) tone signals which are each at -43 dBm or greater and are appropriately separated from the assigned channel frequency and from each other such that the third order mixing of the two interfering signals can occur in non-linear operating regions of elements of the receiver to produce an interfering signal in the frequency band of the spread spectrum signal. However, it has been found that the current operating standards and design specifications promulgated in IS-98 regarding IM distortion are inadequate to assure adequate receiver operation under actual conditions.
One possible solution to this problem is to build additional digital system base stations in order to increase the ratio at the mobile station of desired digital signal strength to IM interference signal strength. However, this certainly would be a very costly and inefficient solution to the problem. Other known possible solutions are to attempt to attenuate the signals (both interfering signals and desired signals) at the receiver front end using one or more fixed signal power attenuators switched on and off according to input signal power levels as a substitute for, or in addition to, initial amplification. However, this solution is often found to be inadequate for at least two reasons. First, the fixed attenuator circuits must be switched on at a high enough received signal power level in order to avoid attenuating the received signal power level below a minimum operating level as the entire fixed amount of attenuation is introduced. Also, fixed attenuator circuits often need to incorporate hysteresis to prevent chattering (i.e., rapid fluctuations, or oscillations, around a fixed point) by switching on the attenuator circuits at an even higher received signal power level. Thus, at the lower end of the received signal power level range where IM signals are most problematic, the received signal is often left unprotected (no attenuation of IM producing interfering signals).
There is, therefore, a need in the industry for a system which addresses these and other related, and unrelated, problems.